The invention described herein relates generally to thermal protection structures for downhole tools and, more particularly, to improved thermal protection structures for heat sensitive downhole electronics used in making geotechnical measurements.
Known devices for thermally protecting electronic components in downhole tools use a Dewar, a double-walled vessel with a removable plug located in one end. A housing surrounds and protects the Dewar and components from the high pressures encountered in geothermal and oil well boreholes. One such device is described in U.S. Pat. No. 4,513,352, "Thermal Protection Apparatus," to G. Bennett et al. Bennett et al. use heat pipes to efficiently conduct heat from heat sensitive components to a heat sink, such as ice, located in a Dewar.
In the Bennett et al. and other known thermal protection devices, a cable having electrical conductors is used to provide the device with power and to carry data signals. The cable can include optical fibers. In known devices, there is an opening in the plug to allow the cable to reach the components protected by the Dewar. The plug and the opening create a significant thermal leak. It is therefore desirable to construct a thermal protection apparatus which reduces thermal losses and which allows electrical communication with the components in a Dewar. Moreover, contact with sensing elements outside of the Dewar and on the opposite side thereof from the downhole cable is made with cabling which passes outside of the Dewar thereby creating a bulky exterior. A more direct electrical and optical fiber connection would be most desirable.